�ݺ�ߣ

Weathering and Soil
Formation

A natural arch produced by erosion
of differentially weathered rock in
Jebel Kharaz (Jordan).

Class -7B
Group C

Weathering and Soil Formation
Roll No.s 15
16
17
18
19
20
21

Group Members

- Dugesh Dubey
- Santanu Gupta
- Tushar Gupta
- Samarth Enosh Harrison
- Mohd. Ibrahim
- Syed Arham Iftekar
- Anugrah Kapoor

Weathering
Weathering is the fracturing or disintegration and
decomposition of rocks by natural agents. It is a static process.
There is no movement of weathered material except where
the pieces fall due to the force of gravity.
Weathering is classified into three types:1. Physical or Mechanical Weathering
2. Chemical Weathering
3. Biological Weathering

Mechanical Weathering
Mechanical weathering is
the physical disintegration
of rocks due to factors like
heat, water and frost. Small
pieces or particles of rock
prise off from the rock
surface. There is no change
in the minerals constituting
the rock. The end result is
soil. Mechanical weathering
is more intense when the
rock layers are exposed on
the surface.

Continued….

Types of Mechanical Weathering
• Block Disintegration: Alternate explosion and
contraction over a long period of time results in
cracks and joints. The rocks then start to split
along the cracks and joints. Slowly the whole
mass of rock splits into blocks. This is known as
block disintegration.
• Exfoliation: The sharp edges of the blocks are
slowly rounded by another process of
mechanical weathering termed exfoliation or
onion peeling.
• Frost shattering: The volume of ice is 1/10 more
than the volume of water and the pressure ice
exerts on the cracks and crevices is very intense.
Slowly angular fragments of rocks prise off along
the crack. Repeated freezing deepens and
widens the cracks. This process is known as frost
shattering.

Chemical Weathering
Chemical weathering is the rotting or decomposition of
rocks, under the influence of atmosphere and water.
This type of weathering is most common in
equatorial and tropical regions where there is
enough moisture and heat to start off chemical
reactions within rock masses.
Acid in groundwater
has weathered
limestone to form
Rusty’s Cave in Dade
County, Georgia.

Types of Chemical Weathering
There are four types of chemical weathering:1.

2.

3.

4.

Oxidation: Oxygen present in the air or water reacts with certain minerals
present in the rock. Oxides form and the rocks slowly crumble ,for example,
iron present I rocks combines with oxygen to form iron oxide or rust.
Carbonation: Carbon dioxide present in the atmosphere dissolves with rain
water to form a weak acid called carbonic acid. This makes rain water
acidic. The acidic rain water reacts with rocks like limestone. Calcium and
bicarbonate ions form which are carried away by the water.
Hydration: This occurs when hydrogen present in water combines with
minerals present in rocks and decomposes it for example, feldspar, a
mineral, is converted to kaolin as a result of hydration.
Solution: Rain water is able to dissolve certain minerals and chemically
weather rocks. Rock salt and gypsum are removed by solution.

Biological Weathering
Biological weathering is a
form of weathering caused
by growth of roots and
burrowing of animals. Plants
roots are the most efficient
agents of the biological
weathering as they give off
acids that contributes to
break down of rocks.
Biological weathering
increases with soil thickness
until the optima for biotic
activity is reached.

Trees put down roots through
joints or cracks in the rock in
order to find moisture. As the
tree grows, the roots gradually
prize the rock apart.

Mass Movement
Mass movement is the
movement of weathered
material down on a slope
due to gravitational
forces. The weathered
material is not
transported by any agent
of erosion. The
movement can be sudden
or vey gradual depending
on the steepness of the
slope, amount of
weathered material and
presence of rain water in
the soil.

Soil Formation
Soil is the thin surface layer on the
earth, comprising mineral particles
formed by the breakdown of
rocks, decayed organic
matter, living organisms, soil water
and soil atmosphere. Soil formation
starts with weathering of rock
masses and accumulation of
weathered particles on the surface
of the rock masses. The
components of soil are found in all
three states of matter.

Soil Profile

A vertical section of the soil from surface down to the
bedrock, showing the different horizons is called the soil
profile. The horizons or layers have different colours.
Generally a soil profile has four horizons:1. Top soil: It contains organic matter and
humus.

2. Subsoil: It contains weathered rock
material, sand, silt and clay. This layer
contains salts and minerals and less
humus.
3. Substratum: It consists of large
fragments of weathered parent rock
material.
4. Bedrock: It provides a base to the three
layers above. The layer is unweathered.

Soil Erosion
Soil forms over thousands
of years but can be
washed away quickly due
to the carelessness of
mankind. The removal of
the fertile top soil by
agents of erosion like rain
water, rivers, wind and
waves is termed as soil
erosion. Sheet erosion
and gully erosion are the
two most common types
of erosion.

Wave cut platform caused by erosion of
cliffs by the sea, at Southerndown in
South Wales.

Soil Conservation
1.
2.
3.
4.

Soil erosion has made soils infertile. Unscientific
methods of cultivation has led to low yields of crops
and washing away of the precious top soil. Soil
conservation means to take measures to protect
the top soil from being eroded. There are several
methods to conserve soil:Contour ploughing
Terrace farming and bunding
Shelter belts
Plugging of Gullies

�ݺ�ߣ

Weathering and Soil Formation Class 7

More Related Content

Weathering and Soil Formation Class 7